Electrical power is generally delivered from an AC (Alternating Current) line power source to a load using conducting wires separated by insulating material. The power source, the electrical load, and the conductors between the power source and an electrical load form a circuit. Any unintended current leakage from any of the conductors in the circuit represents a hazardous condition, which may cause damage to the circuit, electrical shock or fire.
Power cords and extension cords for providing power to refrigerators, air conditioners, dehumidifiers and similar heavy-duty appliances generally have at least three wires (e.g. phase, neutral and ground) insulated from each other. These power cords are often used in environments where they are susceptible to damage. Degradation of the insulation around the wires in these cords may occur due to arcing, fire, overheating, physical or chemical abuse. It is therefore especially important to be able to detect leakage currents in such power cords, and to interrupt the circuit when a leakage current is detected.
Historically, leakage current has been detected and interrupted by fuses, circuit breakers and ground fault circuit interrupters (GFCIs). These types of interrupters are in widespread use in both commercial and residential environments. Fuses and circuit breakers interrupt power to a circuit when an excessive amount of current flows through the circuit for a predetermined period of time. However, the amount of leakage current required to cause shock, or start an electrical fire, can be much lower than the typical current required to burn a fuse or trip a breaker.
GFCIs, which are designed to protect against electrical shock, interrupt power to a circuit when the difference in current flowing through the phase and neutral wires exceeds a predetermined amount. The current difference is equal to the amount of current leaking out of the circuit from the phase or neutral conductors. The trip current of a GFCI, e.g., five milliamps, is typically lower than the amount required to cause shock or start an electrical fire. However, if the leakage current is between the phase and neutral wires of a damaged power cord, there will not be any differential current, but the resulting heat and arcing may be sufficient to start a fire.
Leakage current detector interrupters (LCDIs), can reliably detect leakage current from any conductor in a power cord, and may conveniently be incorporated into either the cord, the plug of that cord, or another suitable location in an electrical system. LCDIs typically consist of three elements: a shield incorporated into the power cord, proximal to all the conductors enclosed therein, so that a portion of the leakage current from any of the conductors will flow in the shield; a detector for detecting current flow in the shield; and a circuit interrupting mechanism that is activated by the detector when a predetermined level of current has flowed in the shield for a predetermined time. The predetermined level of current may be typically in the order of milliamps —much lower than the amount required to cause shock or start an electrical fire.
Circuit interrupters in general may have one of two types of interrupting mechanism. The first type is an intermittent-duty latched relay (known as a mousetrap style mechanism) and the second type is a continuous-duty relay. In a circuit interrupter with a continuous duty relay the main contacts are held closed, electromechanically, by a relay. The relay is normally open when not energized, i.e., when no power is applied to the relay; when energized, that is, when power is applied to the relay, it causes the main contacts to close. When an unwanted condition is detected, power to the relay coil is removed and the relay moves to its normally open condition.